1. Field of the Invention
The present invention relates generally to a projection optical system, and more particularly, to a projection optical system in which brightness of images projected on a screen and brightness uniformity can be improved.
2. Description of the Related Art
Projection optical systems are optical systems in which light emitted from a light source takes the shape of light beams of the required cross-section and aperture, thereby providing uniformity of illumination, and is then projected on a screen via an objective. In recent image projection systems, a micro-mirror element is used as an optical element for forming images. The micro-mirror element is a set of rotary mirrors, for example, a digital micro-mirror device (DMD). The optical system of these image projection systems includes an illumination optical element for homogenous illumination on the set of rotary mirrors and a projection optical element, such as an objective for displaying images on a screen.
As an example of conventional projection optical systems, U.S. Pat. No. 6,688,748 discloses an image projection system including: an illumination source for emitting an illumination beam; a reflective display device for modulating the illumination beam to form a reflected imaging beam; a projection objective group having an optical axis; and a field lens group. Herein, the field lens group is decentered with respect to the optical axis of the projection objective group to redirect the reflected imaging beam along an optical path parallel to the optical axis of the projection objective group. In an embodiment of the invention disclosed in U.S. Pat. No. 6,688,748, the reflective display device is disposed on the optical axis of the projection objective group, and the redirected portion of the optical path of the reflected imaging beam is coincident with the optical axis of the projection objective group. In another embodiment of the invention disclosed in U.S. Pat. No. 6,688,748, the display device is tilted with respect to the optical axis of the projection objective group to accommodate the tilt in focal plane of the projection objective group caused by the redirection of the reflected imaging beam.
However, the drawback of the image projection system is that separation of pupils of projection and illumination elements is difficult due to deficiency of free space in a projection objective near an aperture diaphragm (AD). Due to the problems, the overlap of the pupils occurs and a part of beams is lost. As a result, the brightness of images formed in the image projection system decreases and heterogeneity of the images increases.
As another example of conventional projection optical systems, U.S. Pat. No. 6,439,726 discloses a projection optical system in which light emitted from a light source is directed onto a surface by a first optical element, so that an image is formed which can be detected by a second optical element. The projection optical system includes first, second and third optical parts. The first and second optical parts have an optical axis as a common optical axis and form a second optical element. The light coming from the third optical part is incident on the second optical part at a predetermined angle relative to the common optical axis. At this angle, the third optical part lies outside of an area traversed by the light reflected by the reflecting surface from the second optical part to the first optical part.
However, this projection optical system has problems as follows. Compact projection optical systems have a large diagonal of projected images, while they have a small distance from a projector and a screen. In the system, images are projected outside an optical axis of the optical system for convenience of viewers, so that shadows of the viewers do not fall on the screen. For this purpose, if the projector stands on a table, the screen should be located above the optical axis of the optical system, and if the projector is installed on a ceiling, the screen should be located below the optical axis of the optical system. To generate images on the screen located outside of the optical axis of the optical system, an asymmetric projection objective is used in the optical system. Herein, a set of rotary mirrors is vertically shifted relative to the optical axis of an objective. This is because it is necessary to significantly increase adjustable field of vision. An adjustable area of an object, that is, an area located on the plane of the set of rotary mirrors, can be two and even three times more than useful field of vision equal, as a rule, to the size of the set of rotary mirrors. This causes large complexities in calculation of an objective, and corrections of distortion and aberration. Thus, times and costs in calculation and manufacturing of the optical system increase, and its dimension also increases.